Reflective structures for surface acoustic wave devices

ABSTRACT

Interdigital transducer (IDT) and reflective structure arrangements for surface acoustic wave (SAW) devices are disclosed. Representative SAW devices are described herein with reduced overall size while maintaining good quality factors. In certain embodiments, a SAW device may include an IDT arranged between reflective structures on a piezoelectric material. The reflective structures may include reflective IDTs that are configured to have a phase difference with the IDT to reflect and confine acoustic waves in the piezoelectric material. In certain embodiments, the reflective structures may be electrically connected to at least one of an input signal or an output signal. In this manner, the reflective structures may be configured with reduced size as compared to conventional reflective structures such as gratings, thereby providing a SAW device with reduced dimensions without a negative impact on device performance.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 62/698,515, filed Jul. 16, 2018, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to acoustic wave devices, andparticularly to interdigital transducer (IDT) arrangements for surfaceacoustic wave (SAW) devices.

BACKGROUND

Acoustic wave devices are widely used in modern electronics. At a highlevel, acoustic wave devices include a piezoelectric material in contactwith one or more electrodes. Piezoelectric materials acquire a chargewhen compressed, twisted, or distorted, and similarly compress, twist,or distort when a charge is applied to them. Accordingly, when analternating electrical signal is applied to the one or more electrodesin contact with the piezoelectric material, a corresponding mechanicalsignal (i.e., an oscillation or vibration) is transduced therein. Basedon the characteristics of the one or more electrodes on thepiezoelectric material, the properties of the piezoelectric material,and other factors such as the shape of the acoustic wave device andother structures provided on the device, the mechanical signaltransduced in the piezoelectric material exhibits a frequency dependenceon the alternating electrical signal. Acoustic wave devices leveragethis frequency dependence to provide one or more functions.

Surface acoustic wave (SAW) devices, such as SAW resonators and SAWfilters, are used in many applications such as radio frequency (RF)filters. For example, SAW filters are commonly used in second generation(2G), third generation (3G), and fourth generation (4G) wirelessreceiver front ends, duplexers, and receive filters. The widespread useof SAW filters is due to, at least in part, the fact that SAW filtersexhibit low insertion loss with good rejection, can achieve broadbandwidths, and are a small fraction of the size of traditional cavityand ceramic filters. As the use of SAW filters in modern RFcommunication systems and mobile devices increases, there is a need forSAW filters with good performance characteristics and reduced size.

SUMMARY

The present disclosure relates to acoustic wave devices, andparticularly to interdigital transducer (IDT) and reflective structurearrangements for surface acoustic wave (SAW) devices. Representative SAWdevices are described herein with reduced overall size while maintaininggood quality factors. In certain embodiments, a SAW device may includean IDT arranged between reflective structures on a piezoelectricmaterial. The reflective structures may include reflective IDTs that areconfigured to have a phase difference with the IDT to reflect andconfine acoustic waves in the piezoelectric material. In certainembodiments, the reflective structures may be electrically connected toat least one of an input signal and an output signal. In this manner,the reflective structures may be configured with reduced size ascompared to conventional reflective structures such as gratings, therebyproviding a SAW device with reduced dimensions without a negative impacton device performance.

In one aspect, a SAW device comprises: a piezoelectric material; aninterdigital transducer (IDT) on the piezoelectric material andelectrically connected to an input signal and an output signal; and afirst reflective structure and a second reflective structure on thepiezoelectric material, wherein the IDT is arranged between the firstreflective structure and the second reflective structure; wherein thefirst reflective structure comprises a first reflective IDT and thesecond reflective structure comprises a second reflective IDT. Incertain embodiments, the first reflective IDT and the second reflectiveIDT comprise a phase difference with the IDT. The first reflective IDTand the second reflective IDT may be out of phase with the IDT. Incertain embodiments, the first reflective IDT and the second reflectiveIDT are electrically connected to the input signal and the outputsignal. In other embodiments, the first reflective IDT and the secondreflective IDT are electrically connected to ground and either the inputsignal or the output signal. In certain embodiments, the IDT comprises aplurality of first electrode fingers that are electrically connected tothe input signal and a plurality of second electrode fingers that areelectrically connected to the output signal, and the plurality of firstelectrode fingers are interdigitated with the plurality of secondelectrode fingers. The first reflective IDT may comprise one or morefirst reflective electrode fingers that electrically connected to theinput signal and the second reflective IDT comprises one or more secondreflective electrode fingers that electrically connected to the outputsignal. The one or more first reflective electrode fingers may beinterdigitated with the one or more second reflective electrode fingers.In certain embodiments, a reflective electrode finger of the firstreflective IDT is arranged closest to an electrode finger of the IDT andthe reflective electrode finger and the electrode finger are bothelectrically connected to the same of either the input signal or theoutput signal. In certain embodiments, the SAW device may furthercomprise additional reflective structures, wherein the first reflectiveIDT and the second reflective IDT are configured between the additionalreflective structures and the IDT. The additional reflective structurescomprise reflective gratings, reflective IDTs, or both reflectivegratings and reflective IDTs. In certain embodiments, at least one ofthe IDT, the first reflective IDT, and the second reflective IDTcomprises an apodized IDT.

In another aspect, a SAW device comprises: a piezoelectric material; aninterdigital transducer (IDT) on the piezoelectric material andelectrically connected to an input signal and an output signal; and afirst reflective structure and a second reflective structure on thepiezoelectric material, wherein the IDT is arranged between the firstreflective structure and the second reflective structure; wherein thefirst reflective structure and the second reflective structure areelectrically connected to at least one of the input signal and theoutput signal. In certain embodiments, the first reflective structureand the second reflective structure are electrically connected to bothof the input signal and the output signal. In other embodiments, thefirst reflective structure and the second reflective structure areelectrically connected to ground and at least one of the input signaland the output signal. In certain embodiments, the first reflectivestructure and the second reflective structure comprise reflective IDTsthat have a phase difference with the IDT. In certain embodiments, thefirst reflective structure and the second reflective structure comprisereflective IDTs that are out of phase with the IDT. In certainembodiments, a reflective electrode finger of the first reflectivestructure is arranged closest to an electrode finger of the IDT and thereflective electrode finger and the electrode finger are bothelectrically connected to the same of either the input signal or theoutput signal. The SAW device may further comprise additional reflectivestructures, wherein the first reflective structure and the secondreflective structure are configured between the additional reflectivestructures and the IDT. The additional reflective structures comprisereflective gratings, reflective IDTs, or both reflective gratings andreflective IDTs.

In another aspect, any of the foregoing aspects, and/or various separateaspects and features as described herein, may be combined for additionaladvantage. Any of the various features and elements as disclosed hereinmay be combined with one or more other disclosed features and elementsunless indicated to the contrary herein.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 is a perspective view illustration of a representative surfaceacoustic wave (SAW) device.

FIG. 2A illustrates an example SAW structure that includes aninterdigital transducer (IDT) arranged between two reflectivestructures.

FIG. 2B illustrates an alternative configuration of a SAW structurewhere an IDT is configured between two reflective structures and anumber of reflective fingers within each of the two reflectivestructures is reduced.

FIG. 3A illustrates a SAW structure that includes first and secondreflective structures that comprise reflective IDTs that have a phasedifference with an IDT according to embodiments disclosed herein.

FIG. 3B illustrates a SAW structure that includes an IDT arrangedbetween additional IDTs that are configured to be in phase with the IDT.

FIG. 4A is a graph plotting the admittance of four SAW devices withvarying reflective structures.

FIG. 4B illustrates a zoomed in view of the dashed box of FIG. 4A whereparallel resonant frequencies of the four SAW devices differ.

FIG. 5A is a block diagram of a radio frequency (RF) duplexer thatincludes conventional SAW resonators.

FIG. 5B is a block diagram of an RF duplexer that includes SAWresonators according to embodiments disclosed herein.

FIG. 6A is a top view of a device layout of the RF duplexer of FIG. 5A.

FIG. 6B is a top view of a device layout of the RF duplexer of FIG. 5B.

FIG. 7A is a top view illustration of the TX2 resonator of FIGS. 5A and6A.

FIG. 7B is a top view illustration of the TX2′ resonator of FIGS. 5B and6B.

FIG. 7C is a top view illustration of the TX4 resonator of FIGS. 5A and6A.

FIG. 7D is a top view illustration of the TX4′ resonator of FIGS. 5B and6B.

FIG. 7E is a top view illustration of the TX6 resonator of FIGS. 5A and6A.

FIG. 7F is a top view illustration of the TX6′ resonator of FIGS. 5B and6B.

FIG. 8A is an S-parameters comparison plot representing passbands of theRF duplexer of FIGS. 5A/6A with the RF duplexer of FIGS. 5B/6B.

FIG. 8B is magnified view of frequency range from the S-parameterscomparison plot representing passbands.

FIG. 8C is a Smith chart comparing the antenna reflection impendence ofthe RF duplexer of FIGS. 5A/6A with the RF duplexer of FIGS. 5B/6B.

FIG. 8D is an S-parameters comparison plot representing a zoomed outview of the passbands of the RF duplexer of FIGS. 5A/6A with the RFduplexer of FIGS. 5B/6B.

FIG. 8E is an S-parameters plot comparing return loss of the RF duplexerof FIGS. 5A/6A with the RF duplexer of FIGS. 5B/6B.

FIG. 8F is a Smith chart comparing the TX impendence for the transmit(TX) passband of the RF duplexer of FIGS. 5A/6A with the RF duplexer ofFIGS. 5B/6 B.

FIG. 8G is a comparison plot for duplexer isolation of the RF duplexerof FIGS. 5A/6A with the RF duplexer of FIGS. 5B/6B.

FIG. 8H is a zoomed out view of the comparison plot of FIG. 8G.

FIG. 8I is an S-parameters plot comparing antenna return loss of the RFduplexer of FIGS. 5A/6A with the RF duplexer of FIGS. 5B/6B.

FIG. 9 illustrates a SAW structure that includes reflective structuresthat comprise reflective IDTs according to embodiments disclosed herein.

FIG. 10 illustrates a SAW structure that includes an IDT that isarranged between multiple reflective structures according to embodimentsdisclosed herein.

FIG. 11 illustrates a SAW structure that includes an IDT that isarranged between multiple reflective IDTs according to embodimentsdisclosed herein.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to acoustic wave devices, andparticularly to interdigital transducer (IDT) and reflective structurearrangements for surface acoustic wave (SAW) devices. Representative SAWdevices are described herein with reduced overall size while maintaininggood quality factors. In certain embodiments, a SAW device may includean IDT arranged between reflective structures on a piezoelectricmaterial. The reflective structures may include reflective IDTs that areconfigured to have a phase difference with the IDT to reflect andconfine acoustic waves in the piezoelectric material. In certainembodiments, the reflective structures may be electrically connected toat least one of an input signal and an output signal. In this manner,the reflective structures may be configured with reduced size ascompared to conventional reflective structures such as gratings, therebyproviding a SAW device with reduced dimensions without a negative impacton device performance.

Before describing particular embodiments of the present disclosurefurther, a general discussion of SAW devices is provided. FIG. 1 is aperspective view illustration of a representative SAW device 10. The SAWdevice 10 includes a substrate 12, a piezoelectric layer 14 on thesubstrate 12, an IDT 16 on a surface of the piezoelectric layer 14opposite the substrate 12, a first reflector structure 18A on thesurface of the piezoelectric layer 14 adjacent to the IDT 16, and asecond reflector structure 18B on the surface of the piezoelectric layer14 adjacent to the IDT 16 opposite the first reflector structure 18A.

The IDT 16 includes a first electrode 20A and a second electrode 20B,each of which include a number of electrode fingers 22 that areinterleaved with one another as shown. The first electrode 20A and thesecond electrode 20B may also be referred to as comb electrodes. Alateral distance between adjacent electrode fingers 22 of the firstelectrode 20A and the second electrode 20B defines an electrode pitch Pof the IDT 16. The electrode pitch P may at least partially define acenter frequency wavelength λ of the SAW device 10, where the centerfrequency is the primary frequency of mechanical waves generated in thepiezoelectric layer 14 by the IDT 16. For a single electrode IDT 16 suchas the one shown in FIG. 1 , the center frequency wavelength λ is equalto twice the electrode pitch P. For a double electrode IDT 16, thecenter frequency wavelength λ is equal to four times the electrode pitchP. A finger width W of the adjacent electrode fingers 22 over theelectrode pitch P may define a metallization ratio, or duty factor, ofthe IDT 16, which may dictate certain operating characteristics of theSAW device 10.

In operation, an alternating electrical input signal provided at thefirst electrode 20A is transduced into a mechanical signal in thepiezoelectric layer 14, resulting in one or more acoustic waves therein.In the case of the SAW device 10, the resulting acoustic waves arepredominately surface acoustic waves. As discussed above, due to theelectrode pitch P and the metallization ratio of the IDT 16, thecharacteristics of the material of the piezoelectric layer 14, and otherfactors, the magnitude and frequency of the acoustic waves transduced inthe piezoelectric layer 14 are dependent on the frequency of thealternating electrical input signal. This frequency dependence is oftendescribed in terms of changes in the impedance and/or a phase shiftbetween the first electrode 20A and the second electrode 20B withrespect to the frequency of the alternating electrical input signal. Analternating electrical potential between the two electrodes 20A and 20Bcreates an electrical field in the piezoelectric layer 14 which generateacoustic waves. The acoustic waves travel at the surface and eventuallyare transferred back into an electrical signal between the electrodes20A and 20B. The first reflector structure 18A and the second reflectorstructure 18B reflect the acoustic waves in the piezoelectric layer 14back towards the IDT 16 to confine the acoustic waves in the areasurrounding the IDT 16.

The substrate 12 may comprise various materials including glass,sapphire, quartz, silicon (Si), or gallium arsenide (GaAs) among others,with Si being a common choice. The piezoelectric layer 14 may be formedof any suitable piezoelectric material(s). In certain embodimentsdescribed herein, the piezoelectric layer 14 is formed of lithiumtantalate (LT), or lithium niobate (LiNbO₃), but is not limited thereto.In certain embodiments, the piezoelectric layer 14 is thick enough orrigid enough to function as a piezoelectric substrate. Accordingly, thesubstrate 12 in FIG. 1 may be omitted. Those skilled in the art willappreciate that the principles of the present disclosure may apply toother materials for the substrate 12 and the piezoelectric layer 14. TheIDT 16, the first reflector structure 18A, and the second reflectorstructure 18B may comprise any metal or metal alloy. While not shown toavoid obscuring the drawings, additional passivation layers, frequencytrimming layers, or any other layers may be provided over all or aportion of the exposed surface of the piezoelectric layer 14, the IDT16, the first reflector structure 18A, and the second reflectorstructure 18B. Further, one or more layers may be provided between thesubstrate 12 and the piezoelectric layer 14 in various embodiments.

FIG. 2A illustrates an example SAW structure 24 that includes an IDT 26arranged between two reflective structures 28-1, 28-2. An optionalsubstrate (e.g., 12 of FIG. 1 ) and/or piezoelectric layer or substrate(e.g., 14 of FIG. 1 ) are not shown. The IDT 26 comprises a firstelectrode 30 that is electrically connected to an input signal and asecond electrode 32 that is electrically connected to an output signal.The first electrode 30 comprises a plurality of first electrode fingers30A that are interdigitated with a plurality of second electrode fingers32A of the second electrode 32. As previously described, the pitchbetween the first electrode fingers 30A and the second electrode fingers32A is about equal to a center frequency wavelength A for the SAWstructure 24. While only a certain number of first and second electrodefingers 30A, 32A are illustrated, in practice the IDT 26 can includemany more alternating first and second electrode fingers 30A, 32A.Additionally, each of the reflective structures 28-1, 28-2 comprises aplurality of reflective fingers 28A that may be electrically shorted toone another within each of the respective reflective structures 28-1,28-2. In other configurations, the reflective fingers 28A may beelectrically open to one another. A pitch of the reflective fingers 28Awithin each of the reflective structures 28-1, 28-2 may be configuredsimilar to the pitch between the first electrode fingers 30A and thesecond electrode fingers 32A of the IDT 26. In this manner, reflectionsfrom individual ones of the reflective fingers 28A are in phase for adesired frequency within a resonant cavity between the two reflectivestructures 28-1, 28-2. In operation, a certain number of the reflectivefingers 28A are required to provide adequate confinement within the SAWstructure 24. FIG. 2B illustrates an alternative configuration of anexample SAW structure 34 where the IDT 26 is configured between the tworeflective structures 28-1, 28-2 and the number of reflective fingers28A within each of the two reflective structures 28-1, 28-2 is reduced.In FIG. 2B, the number of the reflective fingers 28A are reduced tothree reflective fingers 28A in each of the reflective structures 28-1,28-2 (from five reflective fingers 28A in FIG. 2A) for illustrativepurposes. In this manner, the alternative configuration of the SAWstructure 34 in FIG. 2B has reduced sized when compared with FIG. 2A;however, device performance may be compromised. In operation, if thenumber of the reflective fingers 28A is too low, a quality factor (Qfactor) of the SAW structure 34 may be too low, indicating a high rateof energy loss. As configured in FIGS. 2A and 2B, the reflectivestructures 28-1, 28-1 may also be referred to as gratings, or gratingreflectors.

According to embodiments disclosed herein, a SAW device may comprise apiezoelectric material, an IDT on the piezoelectric material andelectrically connected to an input signal and an output signal, and afirst reflective structure and a second reflective structure on thepiezoelectric material, wherein the IDT is arranged between the firstreflective structure and the second reflective structure. The firstreflective structure may comprise a first reflective IDT and the secondreflective structure may comprise a second reflective IDT. In certainembodiments, the first reflective IDT and the second reflective IDT areconfigured to have a phase difference or be out of phase with the IDT inorder to provide a resonant cavity with adequate confinement. In thismanner, the first reflective IDT and the second reflective IDT canachieve similar reflection characteristics as larger conventionalreflective structures, thereby providing a SAW device with decreaseddimensions without a decrease in performance characteristics, such as Qfactor.

FIG. 3A illustrates a SAW structure 36 that includes first and secondreflective structures 38-1, 38-2 that comprise reflective IDTs that havea phase difference with the IDT 26 according to embodiments disclosedherein. An optional substrate (e.g., 12 of FIG. 1 ) and/or piezoelectriclayer or substrate (e.g., 14 of FIG. 1 ) are not shown, but may beprovided to form a SAW device. In FIG. 3A, the SAW structure 36 includesthe IDT 26 with the first electrode 30 that is electrically connected tothe input signal and the second electrode 32 that is electricallyconnected the output signal as previously described. The first electrode30 comprises the plurality of first electrode fingers 30A that areinterdigitated with the plurality of second electrode fingers 32A of thesecond electrode 32. In this manner the plurality of first electrodefingers 30A that are electrically connected to the input signal arealternated with the plurality of second electrode fingers 32A that areelectrically connected to the output signal to generate acoustic wavesin response to an electrical signal as previously described. While onlya certain number of the first and second electrode fingers 30A, 32A areillustrated, in practice the IDT 26 can include many more alternatingfirst and second electrode fingers 30A, 32A. Each of the reflectivestructures 38-1, 38-2 comprises a first reflective electrode 40 that iselectrically connected to the input signal and a second reflectiveelectrode 42 that is electrically connected to the output signal. Thefirst reflective electrode 40 includes one or more first reflectiveelectrode fingers 40A, and the second reflective electrode 40 includesone or more second reflective electrode fingers 42A. The one or morefirst reflective electrode fingers 40A are alternated or interdigitatedwith the one or more second reflective electrode fingers 42A to formreflective IDTs. As illustrated in FIG. 3A, the IDT 26 is arrangedbetween the first reflective structure 38-1 and the second reflectivestructure 38-2 such that an individual second reflective electrodefinger 42A is arranged closest to an individual second electrode finger32A of the IDT 26, both of which are electrically connected to theoutput signal. In this manner, the alternating arrangement of electrodefingers 30A, 32A is interrupted by the reflective IDTs of the reflectivestructures 38-1, 38-2. The reflective IDTs are thereby configured tohave a phase difference with the IDT 26 and accordingly, acoustic wavesare reflected and confined within a resonant cavity that is formedbetween the reflective structures 38-1, 38-2 on the piezoelectricmaterial. In certain embodiments, the reflective IDTs are configured tobe out of phase with the IDT 26. Additionally, as the first reflectiveelectrode 40 of each reflective structure 38-1, 38-2 is electricallyconnected to the input signal and the second reflective electrode 42 ofeach reflective structure 38-1, 38-2 is electrically connected to theoutput signal, the reflective structures 38-1, 38-2 may also serve asIDT capacitors that may alter the overall static capacitance. In otherembodiments, the individual second reflective electrode finger 42A isarranged closest to the individual second electrode finger 32A, and bothmay be electrically connected to the input signal to form reflectiveIDTs as previously described. Accordingly, a SAW device may include areflective electrode finger of a reflective IDT that is arranged closestto an electrode finger of an IDT, and the reflective electrode fingerand the electrode finger are both electrically connected to the same ofeither the input signal or the output signal. In contrast to the SAWstructure 36 of FIG. 3A, FIG. 3B illustrates a SAW structure 44 thatincludes the IDT 26 as previously described arranged between additionalIDTs 46-1, 46-2 that are configured to be in phase with the IDT 26. Eachof the additional IDTs 46-1, 46-2 comprises a first electrode 48 withone or more first electrode fingers 48A electrically connected with aninput signal and a second electrode 50 with one or more second electrodefingers 50A electrically connected with an output signal. The firstelectrode fingers 48A and second electrode fingers 50A of the additionalIDTs 46-1, 46-2 are arranged in an alternating manner that continueswith the electrode fingers 30A, 32A of the IDT 26. In this manner, theadditional IDTs 46-1, 46-2 are configured to be in phase with the IDT 26and therefore do not serve to reflect and confine acoustic waves.

FIG. 4A is a graph plotting the admittance of four SAW devices withvarying reflective structures. All of the four SAW devices, SAMPLES 1-4,are configured with a same IDT configured with a same center frequencywavelength λ. SAMPLE 1 is a SAW device that includes a structure similarto the SAW structure 24 of FIG. 2A with an IDT (e.g., 26 of FIG. 2A)length of about 112λ and a reflective structure (e.g., 28-1, 28-2 ofFIG. 2A) length of about 14λ. SAMPLE 2 is a SAW device that includes astructure similar to the SAW structure 34 of FIG. 2B with an IDT (e.g.,26 of FIG. 2B) length of about 112λ and a reflective structure (e.g.,28-1, 28-2 of FIG. 2B) length of about 3λ. SAMPLE 3 is a SAW device thatincludes a structure similar to the SAW structure 36 of FIG. 3A with anIDT (e.g., 26 of FIG. 3A) length of about 112λ and a reflectivestructure (e.g., 38-1, 38-2 of FIG. 3A) length of about 3λ. SAMPLE 4 isa SAW device that includes a structure similar to the SAW structure 44of FIG. 3B with an IDT (e.g., 26 of FIG. 3B) length of about 112λ and areflective structure (e.g., 46-1, 46-2 of FIG. 3B) length of about 3λ.In this manner, SAMPLE 1 includes the IDT in between conventionalreflective gratings, SAMPLE 2 illustrates the performance impact ofreducing the length of conventional gratings, SAMPLE 3 includes the IDTin between reflective IDTs, and SAMPLE 4 illustrates the performanceimpact when adjacent IDTs are in phase with the IDT. The amplitude indecibels (dB) of admittance for each of the samples is plotted withrespect to the frequency of an alternating electrical input signal.Notably, all four samples have a similar series resonant frequency(f_(s)), or resonant frequency. The series resonant frequency representsthe frequency where impedance is minimal. Differences between the foursamples are more prominent at a parallel resonant frequency (f_(p)), orantiresonant frequency. The parallel resonant frequency represents thefrequency where impedance is highest. FIG. 4B illustrates a magnifiedview of the dashed box of FIG. 4A where the parallel resonantfrequencies of each of the samples differ. As illustrated, when thelength of conventional gratings are reduced from a value of 14λ forSAMPLE 1 to a value of 3λ for SAMPLE 2, the parallel resonant frequencyhas a higher dB value which indicates the conventional gratings are lessreflective with decreased length and therefore have a negative impact onthe Q factor. In contrast, the parallel resonant frequency for SAMPLE 3has a similar dB value to SAMPLE 1, indicating a similar reflectanceperformance and without a negative impact on the Q factor. The parallelresonant frequency for SAMPLE 3 does show a minor shift in frequencywhich indicates the reflective IDT structures of SAMPLE 3 have a minorcoupling impact on bandwidth. This may be due to a minor increase instatic capacitance as the reflective structure (e.g., 38-1, 38-2 of FIG.3A) may also function as an IDT capacitor. SAMPLE 4 illustrates thelargest change in the dB value at the parallel resonant frequencydemonstrating the worst reflectance performance and largest impact onthe Q factor.

SAW devices according to embodiments disclosed herein may beincorporated within larger devices and systems to provide simplifiedlayouts or topologies. FIGS. 5A, 5B, 6A, and 6B illustraterepresentative radio frequency (RF) duplexing devices with various SAWdevices as disclosed herein. RF duplexing devices typically areconfigured to receive signals and transmit signals of a different bandusing a common antenna. One of the primary challenges of duplexing isthat RF transmission signals and RF receive signals can interfere withone another and accordingly, RF duplexing devices may employ one or morefilters to improve isolation.

FIG. 5A is a block diagram of an RF duplexer 52 that includesconventional SAW resonators. The RF duplexer 52 includes a transmit (TX)port, a receive (RX) port, and an antenna (ANT) port. A TX filter 54 ispositioned between the TX port and the antenna port, and an RX filter 56is positioned between the RX port and the antenna port. The TX filter 54is configured as a ladder filter with series resonators TX1, TX3, TX5and shunt resonators TX2, TX4, TX6, all of which may be configured witha structure similar to the SAW structure 24 of FIG. 2A. The RX filter 56includes series resonators RX1, RX3, a shunt resonator RX2, a SAWcoupled resonator filter (CRF) structure (5-IDT CRF) that includes fiveIDTs that alternate between input IDTs and output IDTs, and a capacitor58 that is connected between the input and output of the 5-IDT CRF. FIG.5B is a block diagram of an RF duplexer 60 that includes SAW resonatorsaccording to embodiments disclosed herein. The RF duplexer 60 includesthe same RX filter 54 of FIG. 5A between the TX port and the antennaport, but a different TX filter 62. In particular, the TX filter 62includes shunt resonators TX2′, TX4′, TX6′ that are configured with astructure similar to the SAW structure 36 of FIG. 3A. In this regard,the shunt resonators TX2′, TX4′, TX6′ are configured with reflectiveIDTs that provide reduced die size without significant loss ofperformance.

FIG. 6A is a top view of a device layout of the RF duplexer 52 of FIG.5A. As illustrated, the RF duplexer 52 includes the resonators TX1 toTX6, the resonators RX1 to RX3, the 5-IDT CRF, and the capacitor 58 aspreviously described as well as areas for RX, TX, antenna, and variousground connections. FIG. 6B is a top view of a device layout of the RFduplexer 60 of FIG. 5B. As illustrated, the RF duplexer 60 includes theresonators TX2′, TX4′, TX6′ in addition to the resonators TX1, TX3, TX5,the resonators RX1 to RX2, and the 5-IDT CRF, and the capacitor 58 aspreviously described as well as areas for RX, TX, antenna, and variousground connections. Due to the configuration of the resonators TX2′,TX4′, TX6′ there is noticeably improved die space savings in theseareas.

FIG. 7A is a top view illustration of the TX2 resonator of FIGS. 5A and6A, and FIG. 7B is a top view illustration of the TX2′ resonator ofFIGS. 5B and 6B. In a non-limiting example, the TX2 resonator comprisesa length 64 of about 716.6 microns (μm) and a width 66 of about 141.2 μmwhile the TX2′ resonator comprises a length 68 of about 611.6 μm and awidth 70 of about 141.2 μm. The size reduction from the length 64 of theTX2 resonator to the length 68 of the TX2′ resonator is due to replacingconventional grating reflectors with reflective IDTs as previouslydescribed. In this manner, the TX2′ resonator demonstrates about a14.65% reduction in die area. FIG. 7C is a top view illustration of theTX4 resonator of FIGS. 5A and 6A, and FIG. 7D is a top view illustrationof the TX4′ resonator of FIGS. 5B and 6B. In a non-limiting example, theTX4 resonator comprises a length 72 of about 777.7 μm and a width 74 ofabout 93.9 μm while the TX4′ resonator comprises a length 76 of about683.5 μm and a width 78 of about 93.9 μm. In this manner, the TX4′resonator demonstrates about a 12.11% reduction in die area. FIG. 7E isa top view illustration of the TX6 resonator of FIGS. 5A and 6A, andFIG. 7F is a top view illustration of the TX6′ resonator of FIGS. 5B and6B. In a non-limiting example, the TX6 resonator comprises a length 80of about 652.8 μm and a width 82 of about 149.6 μm while the TX6′resonator comprises a length 84 of about 555.1 μm and a width 86 ofabout 149.6 μm. In this manner, the TX6′ resonator demonstrates about a14.966% reduction in die area. While the specific dimensions listedabove are provided, relative resonator sizes may be dependent on thetarget frequency band that a specific resonator is configured tooperate. In this regard, replacing conventional grating reflectors withIDT reflectors as described herein can save significant die area in SAWresonators configured for various operating bands.

FIGS. 8A to 8I are simulation plots comparing the performance of the RFduplexer 52 of FIGS. 5A/6A with the RF duplexer 60 of FIGS. 5B/6B. InFIGS. 8A to 8I, Duplexer 1 refers to the RF duplexer 52 of FIGS. 5A/6Aand Duplexer 2 refers to the RF duplexer 60 of FIGS. 5B/6B. Theperformance comparisons of Duplexer 1 and Duplexer 2 are useful todemonstrate that SAW devices, and in particular SAW resonators, asdisclosed herein may have reduced die size without a significant impacton device performance. FIG. 8A is an S-parameters comparison plotrepresenting passbands of Duplexer 1 and Duplexer 2. The S-parametermagnitude is plotted in decibels (dB) across a megahertz (MHz) frequencyrange. Insertion loss, or S2,1, is an indication of how much power istransferred between the TX port and the antenna port. For frequencieswhere S2,1 is at or near 0 dB, then substantially all power from asignal is transferred. Accordingly, a TX passband is illustrated wherethe S2,1 values are at or near 0 dB. On either side of the TX passband,or the shoulder regions, the S2,1 values decrease rapidly. As the S2,1value becomes farther away from 0 dB, more and more power is rejected.As illustrated, the TX passbands for Duplexer 1 and Duplexer 2 aresubstantially the same. FIG. 8B is an S-parameters comparison plotrepresenting a magnified view of the TX passbands of FIG. 8A. In FIG.8B, the comparison plot highlights the frequency range of 778 MHz to 808MHz from FIG. 8A. FIG. 8C is a Smith chart comparing the antennareflection impedance for the TX passband of Duplexer 1 and Duplexer 2.The chart illustrates the reflection scattering parameter (S1,1) at theantenna port in the TX passband frequency. Values at or near 1.0 in thecenter of the plot indicate the signal frequencies are passing throughthe TX filter 54. FIG. 8D is an S-parameters comparison plotrepresenting a zoomed out view of the TX passbands of FIG. 8A. In FIG.8D, the comparison plot illustrates a large frequency range from 100 MHzto 6000 MHz to show a similar rejection performance for frequencies welloutside of the TX passbands. FIG. 8E is an S-parameters plot comparingreturn loss of Duplexer 1 and Duplexer 2. Return loss is an indicationof voltage reflection, and S2,2 represents how much power is reflectedat the TX port. For frequencies where S2,2 is at or near 0 dB, thensubstantially all power from a signal is reflected by the TX filter 54.FIG. 8F is a Smith chart comparing the TX impendence for the TX passbandof Duplexer 1 and Duplexer 2. The chart illustrates the reflectionscattering parameter (S2,2) at the RX port in the TX passband frequency.FIG. 8G is a comparison plot for duplexer isolation in dB for Duplexers1 and 2, where a lower dB value indicates better isolation. S3,2parameter values are plotted for Duplexers 1 and 2 to show isolationbetween the RX port and the TX port. S3,2 values in the frequency rangeof about 756 MHz to about 770 MHz indicate how much power may be leakingfrom the TX port to the RX port, and S3,2 values in the frequency rangeof about 786 MHz to about 800 MHz indicate how much power may be leakingfrom the RX port to the TX port. As illustrated, the dB values are at ornear −60 dB for each of these frequency ranges, indicating goodisolation and low power leakage. FIG. 8H is a zoomed out view of thecomparison plot of FIG. 8G. In FIG. 8H, the comparison plot illustratesa large frequency range from 100 MHz to 6000 MHz to show a similarisolation performance for frequencies well outside of the TX passbands.FIG. 8I is an S-parameters plot comparing antenna return loss ofDuplexer 1 and Duplexer 2. As illustrated, the antenna return loss,plotted as S1,1, is similar between Duplexer 1 and Duplexer 2.

FIG. 9 illustrates a SAW structure 88 that includes reflectivestructures 90-1, 90-2 that comprise reflective IDTs according toembodiments disclosed herein. An optional substrate (e.g., 12 of FIG. 1) and/or piezoelectric layer or substrate (e.g., 14 of FIG. 1 ) are notshown, but may be provided to form a SAW device. In FIG. 9 , the SAWstructure 88 includes the IDT 26 with the first electrode 30 that iselectrically connected to the input signal and the second electrode 32that is electrically connected the output signal as previouslydescribed. The first electrode 30 comprises the plurality of firstelectrode fingers 30A that are interdigitated with the plurality ofsecond electrode fingers 32A of the second electrode 32. In this mannerthe plurality of first electrode fingers 30A that are electricallyconnected to the input signal are alternated with the plurality ofsecond electrode fingers 32A that are electrically connected to theoutput signal to generate acoustic waves in response to an electricalsignal as previously described. As with previous embodiments, while onlya certain number of the first and second electrode fingers 30A, 32A areillustrated, in practice the IDT 26 can include many more alternatingfirst and second electrode fingers 30A, 32A. Each of the reflectivestructures 90-1, 90-2 comprises a first reflective electrode 92 that iselectrically connected to ground and a second reflective electrode 94that is electrically connected to the output signal. The firstreflective electrode 92 includes one or more first reflective electrodefingers 92A and the second reflective electrode 94 includes one or moresecond reflective electrode fingers 94A. The one or more firstreflective electrode fingers 92A are alternated or interdigitated withthe one or more second reflective electrode fingers 94A to formreflective IDTs. As illustrated in FIG. 9 , the reflective structures90-1, 90-2 may be arranged on opposing sides of the IDT 26 such that anindividual second reflective electrode finger 94A is arranged closest toan individual second electrode finger 32A of the IDT 26, both of whichare electrically connected to the output signal. In this manner, thealternating arrangement of electrode fingers 30A, 32A is interrupted bythe reflective IDTs of the reflective structures 90-1, 90-2. Thereflective IDTs are thereby configured to have a phase difference, andin some embodiments be out of phase with the IDT 26 and accordingly,acoustic waves are reflected and confined within a resonant cavity thatis formed between the reflective structures 90-1, 90-2 on thepiezoelectric material. In other embodiments, the individual secondreflective electrode finger 94A is arranged closest to the individualsecond electrode finger 32A, and both may be electrically connected tothe input signal to form reflective IDTs as previously described.Additionally, as each reflective structure 90-1, 90-2 is electricallyconnected to ground and at least one of the input signal or the outputsignal, the reflective structures 90-1, 90-2 may also serve as IDTcapacitors that may alter the overall static capacitance.

FIG. 10 illustrates a SAW structure 96 that includes an IDT 98 that isarranged between multiple reflective structures according to embodimentsdisclosed herein. An optional substrate (e.g., 12 of FIG. 1 ) and/orpiezoelectric layer or substrate (e.g., 14 of FIG. 1 ) are not shown,but may be provided to form a SAW device. In FIG. 10 , the SAW structure98 includes the IDT 98 with a first electrode 100 that is electricallyconnected to the input signal and a second electrode 102 that iselectrically connected to the output signal as previously described. Thefirst electrode 100 comprises a plurality of first electrode fingers100A that are interdigitated with a plurality of second electrodefingers 102A of the second electrode 102. In this manner the pluralityof first electrode fingers 100A that are electrically connected to theinput signal are alternated with the plurality of second electrodefingers 102A that are electrically connected to the output signal togenerate acoustic waves in response to an electrical signal aspreviously described. As with previous embodiments, while only a certainnumber of first and second electrode fingers 100A, 102A are illustrated,in practice the IDT 98 can include many more alternating first andsecond electrode fingers 100A, 102A. In addition to first and secondreflective structures 104-1, 104-2, additional reflective structure106-1, 106-2 are arranged on either side of the IDT 98. Each of thefirst and second reflective structures 104-1, 104-2 comprises a firstreflective electrode 108 that is electrically connected to the inputsignal and a second reflective electrode 110 that is electricallyconnected to the output signal. In other embodiments, at least one ofthe first reflective electrode 108 and the second reflective electrode110 may be electrically connected to ground while the other of the firstreflective electrode 108 and the second reflective electrode 110 may beelectrically connected to either the input signal or the output signal.The first reflective electrode 108 includes one or more first reflectiveelectrode fingers 108A that are alternated or interdigitated with one ormore second reflective electrode fingers 110A of the second reflectiveelectrode 110 to form reflective IDTs as previously described. Anindividual second reflective electrode finger 110A is arranged closestto an individual second electrode finger 102A of the IDT 98 and both areelectrically connected to the output signal, or the input signal inother embodiments. In this regard, the reflective structures 104-1,104-2 form reflective IDTs that have a phase difference with the IDT 98,and acoustic waves may be reflected and confined within a resonantcavity that is formed between the first and second reflective structures104-1, 104-2 on the piezoelectric material. In certain embodiments, thereflective IDTs are out of phase with the IDT 98. The additionalreflective structures 106-1, 106-2 are configured as reflective gratingsto further reflect and confine acoustic waves within the resonantcavity. As illustrated in FIG. 10 , the reflective structures 104-1,104-2 are configured between the additional reflective structures 106-1,106-2 and the IDT 98. In other embodiments, the order may be reversedsuch that the additional reflective structures 106-1, 106-2 areconfigured between the reflective structures 104-1, 104-2 and the IDT98.

FIG. 11 illustrates a SAW structure 112 that includes an IDT 114 that isarranged between multiple reflective IDTs according to embodimentsdisclosed herein. An optional substrate (e.g., 12 of FIG. 1 ) and/orpiezoelectric layer or substrate (e.g., 14 of FIG. 1 ) are not shown,but may be provided to form a SAW device. In FIG. 11 , the SAW structure112 includes the IDT 114 with a first electrode 116 that is electricallyconnected to the input signal and a second electrode 118 that iselectrically connected the output signal as previously described. Thefirst electrode 116 comprises a plurality of first electrode fingers116A that are interdigitated with a plurality of second electrodefingers 118A of the second electrode 118. In this manner the pluralityof first electrode fingers 116A that are electrically connected to theinput signal are alternated with the plurality of second electrodefingers 118A that are electrically connected to the output signal togenerate acoustic waves in response to an electrical signal aspreviously described. As with previous embodiments, while only a certainnumber of the first and second electrode fingers 116A, 1118A areillustrated, in practice the IDT 114 can include many more alternatingfirst and second electrode fingers 116A, 118A. First reflectivestructures 120-1, 120-2 and second reflective structures 122-1, 122-2are arranged on either side of the IDT 114. In certain embodiments, eachof the first reflective structures 120-1, 120-2 is arranged between thesecond reflective structures 122-1, 122-2. As illustrated in FIG. 11 ,the first reflective structures 120-1, 120-2 and the second reflectivestructures 122-1, 122-2 are configured as reflective IDTs that are havea phase difference with the IDT 114 to reflect and confine acousticwaves as previously described. In certain embodiments, the reflectiveIDTs are out of phase with the IDT 114. In certain embodiments, thefirst reflective structures 120-1, 120-2 and the second reflectivestructures 122-1, 122-2 are each electrically connected to both theinput signal and the output signal. In other embodiments, the firstreflective structures 120-1, 120-2 and the second reflective structures122-1, 122-2 may each be electrically connected to ground and either theinput signal or the output signal. The first reflective structures120-1, 120-2 and the second reflective structures 122-1, 122-2 may beconfigured to reflect different frequency bands and may comprisedifferent numbers of alternating electrodes fingers. In furtherembodiments, the SAW structure 112 may comprise additional reflectivestructures, including reflective IDTs and reflective gratings to furtherreflect and confine acoustic waves.

For simplicity, all of the embodiments disclosed herein are illustratedwith unapodized IDTs where all of the IDT electrode fingers have auniform length. In certain embodiments, one or more of the IDTs andreflective IDTs as previously described may comprise an apodized IDTwhere electrode fingers have different lengths at different positionsalong the apodized IDT that are configured for a particular responsefunction. In certain embodiments, one or more of the IDTs as previouslydescribed may comprise a metallization ratio, or duty factor, of anyrange between 0 and 1 of a center wavelength λ. In certain embodiments,the metallization ration is in a range of about 0.1 to about 0.9; or ina range of about 0.2 to about 0.8; or in a range of about 0.3 to about0.7; or in a range of about 0.4 to about 0.5. In certain embodiments,the metallization ratio comprises a value of about 0.4, or a value ofabout 0.5.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A surface acoustic wave (SAW) device, comprising:a piezoelectric material; an interdigital transducer (IDT) on thepiezoelectric material and electrically connected to an input signal andan output signal; and a first reflective structure and a secondreflective structure on the piezoelectric material, wherein the IDT isarranged between the first reflective structure and the secondreflective structure to form a resonant cavity that is bounded by thefirst reflective structure and the second reflective structure; whereinthe first reflective structure comprises a first reflective IDT and thesecond reflective structure comprises a second reflective IDT.
 2. TheSAW device of claim 1, wherein the first reflective IDT and the secondreflective IDT comprise a phase difference with the IDT.
 3. The SAWdevice of claim 1, wherein the first reflective IDT and the secondreflective IDT are out of phase with the IDT.
 4. The SAW device of claim1, wherein the first reflective IDT and the second reflective IDT areelectrically connected to the input signal and the output signal.
 5. TheSAW device of claim 1, wherein the first reflective IDT and the secondreflective IDT are electrically connected to ground and either the inputsignal or the output signal.
 6. The SAW device of claim 1, wherein theIDT comprises a plurality of first electrode fingers that areelectrically connected to the input signal and a plurality of secondelectrode fingers that are electrically connected to the output signal,and the plurality of first electrode fingers are interdigitated with theplurality of second electrode fingers.
 7. The SAW device of claim 6,wherein the first reflective IDT comprises one or more first reflectiveelectrode fingers that electrically connected to the input signal andthe second reflective IDT comprises one or more second reflectiveelectrode fingers that electrically connected to the output signal. 8.The SAW device of claim 7, wherein the one or more first reflectiveelectrode fingers are interdigitated with the one or more secondreflective electrode fingers.
 9. The SAW device of claim 1, wherein areflective electrode finger of the first reflective IDT is arrangedclosest to an electrode finger of the IDT and the reflective electrodefinger and the electrode finger are both electrically connected to thesame of either the input signal or the output signal.
 10. The SAW deviceof claim 1, further comprising additional reflective structures, whereinthe first reflective IDT and the second reflective IDT are configuredbetween the additional reflective structures and the IDT.
 11. The SAWdevice of claim 10, wherein the additional reflective structurescomprise reflective gratings.
 12. The SAW device of claim 10, whereinthe additional reflective structures comprise reflective IDTs.
 13. TheSAW device of claim 10, wherein the additional reflective structurescomprise reflective gratings and reflective IDTs.
 14. The SAW device ofclaim 1, wherein at least one of the IDT, the first reflective IDT, andthe second reflective IDT comprises an apodized IDT.
 15. A surfaceacoustic wave (SAW) device, comprising: a piezoelectric material; aninterdigital transducer (IDT) on the piezoelectric material andelectrically connected to an input signal and an output signal; and afirst reflective structure and a second reflective structure on thepiezoelectric material, wherein the IDT is arranged between the firstreflective structure and the second reflective structure to form aresonant cavity that is bounded by the first reflective structure andthe second reflective structure; wherein the first reflective structureand the second reflective structure are electrically connected to atleast one of the input signal or the output signal.
 16. The SAW deviceof claim 15, wherein the first reflective structure and the secondreflective structure are electrically connected to both of the inputsignal and the output signal.
 17. The SAW device of claim 15, whereinthe first reflective structure and the second reflective structure areelectrically connected to ground and at least one of the input signal orthe output signal.
 18. The SAW device of claim 15, wherein the firstreflective structure and the second reflective structure comprisereflective IDTs that have a phase difference with the IDT.
 19. The SAWdevice of claim 15, wherein the first reflective structure and thesecond reflective structure comprise reflective IDTs that are out ofphase with the IDT.
 20. The SAW device of claim 19, wherein a reflectiveelectrode finger of the first reflective structure is arranged closestto an electrode finger of the IDT and the reflective electrode fingerand the electrode finger are both electrically connected to the same ofeither the input signal or the output signal.
 21. The SAW device ofclaim 15, further comprising additional reflective structures, whereinthe first reflective structure and the second reflective structure areconfigured between the additional reflective structures and the IDT. 22.The SAW device of claim 21, wherein the additional reflective structurescomprise reflective gratings.
 23. The SAW device of claim 21, whereinthe additional reflective structures comprise reflective IDTs.
 24. TheSAW device of claim 21, wherein the additional reflective structurescomprise reflective gratings and reflective IDTs.